The present invention relates to an apparatus for growing single crystals of dissociative semi-conducting compounds such as GaAs, InAs, GaP and InP, which are suitable for the production of the substrates of IC devices and optical devices.
In the production of single crystals of dissociative compounds, it is important to control the stoichiometry of the single crystals. If there is some deviation in the stoichiometry in the melt material for producing a single crystal, point defects are introduced into the crystal. The point defects aggregate and grow to dislocation loops in the crystal. Furthermore, the point defects may possibly reduce the resistance against plastic deformation of the crystal and also increase the dislocation density. It is known that the thus formed lattice defects have significant adverse effects, for instance, on the performance of IC devices using such single crystals. In order to most appropriately control the stoichiometry of the melt materials for producing a single crystal, it is considered to be effective to grow the single crystal, with the melt equilibrated with a volatile component gas at an optimum pressure. For an apparatus employed for this purpose, a technique of sealing the volatile component gas at high temperatures is supremely important. By taking as an example a dissociative compound, GaAs, the importance of the technique will now be explained. Conventionally, a GaAs single crystal is produced by Horizontal Bridgman Method (hereinafter referred to as the HB method) or by Liquid Encapsulated Czochralski method (hereinafter referred to as the LEC method). These conventional methods, however, have the following shortcomings and cannot sufficiently meet the following requirements. For instance, round semi-insulating wafers with (100) surfaces are required for use in IC substrates. The HB method in which a single crystal is grown in a boat with the GaAs melt equilibrated with an arsenic gas has the advantage that the defect density may be controlled. However, it has the shortcoming that since an ingot with a &lt;111&gt; direction is grown in a quartz boat in the method, the ingot must be cut into a wafer at an angle of 54.7.degree. with respect to the growing direction of the crystal and then the wafer must be cut out into a round shape. These processes are not only extra ones to be eliminated, but also can cause an inhomogeneity of impurities and defects within the wafer. Furthermore, in the HB method, a quartz ampule containing the boat must be sealed by welding before the growth process and it must be then cut again in order to take out the crystal therefrom.
In the LEC method, however, in order to avoid dissociative loss of As, the entire surface of the GaAs melt in a crucible is sealed with a B.sub.2 O.sub.3 liquid. In this method, it is difficult to optimize the crystal growth so as to minimize the density of point defects. In other words, because the fraction of the constituent elements is decided by the composition of the source material synthesized, precise control of the melt stoichiometry during the crystal growth is impossible, so that a number of point defects are apt to be introduced into the crystal due to the non-stoichiometry. The inhomogeneity of the defect density is inevitable in the growth direction. Furthermore, in this method, the temperature gradient near the liquid-solid interface cannot be reduced significantly, since the dissociative loss of As from the crystal surface takes place. For this reason, the dislocation density increases due to the thermal strain during the cooling process.
In order to eliminate the shortcomings of these conventional methods, it is necessary to adopt a pulling method which allows a single crystal to grow in a &lt;100&gt; direction at a controlled arsenic pressure. Methods to pull single crystals in a sealed chamber were reported by P. C. Leung and W. P. Allred in the Journal of Crystal Growth, Vol. 19 (1973), pages 356-358, by M. Moulin, M. Faure and G. Bichon in the Journal of Crystal Growth 24/25 (1974), pages 376-379 and by J. B. Mullin, W. R. Macewan, C. H. Holliday and A. E. V. Webb in the Journal of Crystal Growth 13/14 (1972), pages 629-634. The method in the first report, however, has the shortcoming that a growth chamber, in which an arsenic gas is sealed, must be cut after the growth process in order to take out the crystal therefrom as in the HB method. In the method described in the second and third reports, the growth chamber can be divided at a taper joint. However, a tight taper joint, if it is applied to a seal portion with a large diameter, the joint will become very difficult to divide again. Furthermore, since the growth chambers in the above reports are made of quartz which is softened when heated to 1240.degree. C. (the melting point of GaAs), the chamber may be unfavorably deformed if the pressure difference becomes large across the chamber wall. The above-mentioned shortcomings make it difficult to scale up the size of the growth chamber so as to produce single crystals with a large diameter.
It seems that these shortcomings hindered these apparatus from being developed to an industrial one, although these methods had the advantage over the LEC method.
Thus, there is a strong industrial demand for development of a crystal pulling apparatus which is capable of growing single crystals, semi-insulating or conductive and with a large diameter, at a controlled arsenic pressure. Such an apparatus should have a growth chamber with the following features:
(1) It can retain its mechanical strength and gastightness at temperatures as high as 1300.degree. C. in a volatile component gas atmosphere.
(2) It can be divided without destruction and the divided portion of the chamber can be sealed tightly without difficulty, so that the chamber can be used repeatedly.
(3) It has an optical window through which the growing state of the crystal can be observed from outside during the course of the growth.
(4) It has rotating seals through which rotation is transmitted to the growing crystal and the crucible.
(5) It has a particular portion whose temperature is kept not only constant, but also lowest in the whole chamber-wall so that the arsenic pressure in the chamber may be controlled by the temperature of the particular portion.